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Essentially, it's a one-on-one card duel. Each player is represented by a "hero," who has 30 health points and 30 cards. Most cards generate "minions," who can attack your opponent or defend your hero. Other cards allow you to attack enemies directly, make minions more powerful or recover lost health points. If you can whittle down the competition's health points to zero, you win.

It's a virtual version of a collectible card game like Magic: The Gathering or Pokémon. Win a few games and you're rewarded with more powerful cards to add to your deck. The basic decks provided with the game are good enough for a few practice games, but you'll want to start building your own after you get crushed by online competitors.

And that's where Hearthstone leaps from a 15-minute diversion to an obsession. There are more than 450 cards available, some of which you have to create using a clever crafting system. There are nine heroes — mages, warriors, rogues and so on — each of whom has cards only he or she may use. You'll want to spend time playing as each character and then, once you've picked a favorite, build the optimal 30-card deck.

If you're worried that your deck will never match up to that of a Hearthstone veteran, try visiting the Arena. There, each player is forced to assemble a fresh deck from randomly selected cards, adding a bit more luck to the equation.

Hearthstone has been available for a few months on PC and Mac, but the just-released iPad version shines particularly bright. The illustrations on the cards are gorgeous, and the animations and sound effects are elegant and effective. It's a beautifully executed labor of love from a company that usually works on a much bigger canvas.

And it's free.

Yes, you'll need to spend some money if you expect to track down some of the more rare cards, but you can expect dozens of hours of enjoyment from Hearthstone without investing one cent.